This invention relates to substances known to change their light absorption characteristics when irradiated with light of different wavelengths.
Photochromism is a reversible change in the light absorption properties of a substance when the substance is irradiated with light of a different wavelength. Typically, irradiation with ultraviolet (UV) light will cause a photochromic substance to absorb visible light (become colored). The irradiating light can be either monochromatic or polychromatic. When the irradiating light is removed, the substance returns to a colorless state. These photochromic compounds have applications in a variety of fields, the most well known being eyeglasses that darken outdoors (in the UV and visible light of the sun) and return to a transparent state indoors. Other uses can be UV driven filters (optical switches), display elements, or optical recording media, for example. One such optical switch can take the form of a liquid-filled fiber-optic, for example.
Photochromism can occur through a variety of photochemical mechanisms. A discussion of many of these mechanisms is given in a review article by Henri Bouas-Laurent And Heinz Dürr.1 
Some of the mechanisms for reversible photochromic reactions discussed by Bouas-Laurent and Dürr, and typical compound types that exhibit these mechanisms, are shown in Table 1.
TABLE 1Reaction MechanismRepresentative Compound TypePericyclic reactionsSpiropyransCis-trans isomerizationAzo compoundsIntramolecular hydrogen transferAnilsIntramolecular group transferPolycyclic quinonesDissociation processes (bond cleavage)TriarylmethanesElectron transferViologens
Photochromic molecules such as these relax back to a colorless form at different rates, depending on the chemistry involved. In general the back reaction rates are not externally adjustable. Obtaining a different relaxation rate usually requires the synthesis of a molecule with a modified structure. The relaxation lifetimes of photochromic molecules must often be different for different applications. For example, relaxation rates on the order of seconds may be desired for the steady state build up of coloration in a low light flux environment like sunlight, while much shorter relaxation rates would be desired for optical switches for communications.